The anionic polymerization of conjugated diene and 410 vinylaromatic monomers in the presence of metal-alkyl and/or metal-aryl catalysts, resulting in so-called "living polymers" is a well-known technique and is described e.g. by M Schwarc in "Carbanions, Living Polymers and E1. Transfer Processes", Interscience Publisher, J Wiley and Sons, New York, 1956.
With this living polymer technique it is possible, in particular, to prepare both linear and branched block copolymers of poly(conjugated dienes), such as polybutadiene and/or polyisoprene and poly(vinylaromatic), such as poly(styrene).
Using this technique, linear or branched block copolymers can be prepared by coupling intermediate living block copolymers or block segments with a difunctional or polyfunctional coupling agent or linear block copolymers can be prepared by fully sequential polymerization of the before-mentioned monomers by using a monofunctional or difunctional alkali-metal alkyl initiator and more in particular mono- or dilithium-alkyl or lithium-aryl initiators.
An advantage of the latter sequential polymerization methods is formed by the absence of an uncontrolled amount of diblock copolymer. Said diblock is obtained in the case of preparation of linear triblock copolymer or branched block copolymer by coupling of intermediately prepared living copolymers, due to the normally incomplete coupling reaction and subsequent termination of the obtained block copolymer mixture by means of treatment with a proton donating reagent such as alcohol, acid or water.
It is generally known in the art that such terminated intermediate uncoupled block copolymers or block segments have a negative impact on several physical properties of the finally used compositions, in which said block copolymers are incorporated, whereas on the other hand the melt viscosity of such compositions will be lowered, which is attractive for processing.
Due to the increasing requirements of modern processing techniques for the major applications of said block copolymer compositions, a strong need for high quality, standardized block copolymers arose during the last decennium.
It will be appreciated that the manufacture of fully sequential polymerized linear block copolymers and more in particular linear triblock copolymers, using a mono-functional alkalimetal initiator, has developed as the most preferred embodiment of the three before-mentioned options, as the obtained block copolymers could meet these processing technique requirements by mixing them if desired with predetermined, standardized amounts of smaller block copolymers and could provide moreover to the finally used compositions an optimal combination of physical properties.
In particular in the field of block copolymer containing adhesive compositions there is still a strong need to achieve a good balance of rheological characteristics and physical adhesive and mechanical properties of said end use compositions.
For example, the well-known three block copolymers of the type A-B-A, wherein the A-blocks represent poly(vinyl aromatic) and the B-block represents poly(conjugated diene) and more in particular poly(isoprene) normally show satisfactory adhesive properties and mechanical properties but poor thermal resistance. In addition their rather high viscosity in the molten compositions which are preferably used for end use processing processes due to environmental aspects, creates problems in the processing and transformation thereof.
Therefore it was a primary object of the present invention to provide block copolymers, which should meet the modern processing requirements of end use processing and which should show an excellent balance of rheological characteristics and physical adhesive and mechanical properties of said end use compositions. Another related object of the present invention was to provide a process for the economically attractive manufacture of said block copolymers.